Britain’s National Security Council unanimously backed action against Syria in response to a suspected chemical weapons attack, Prime Minister David Cameron said on Wednesday, a day after he set out the case for a targeted military strike.

Media companies including The New York Times, Twitter and The Huffington Post lost control of some of their websites on Tuesday after hackers supporting the Syrian government breached the Australian internet company that manages their addresses.

Tomahawk cruise missiles are likely to be launched at night against hundreds of Syrian targets, including some of President Bashar al-Assad's elite military units, if the US and allies launch a military strike in retaliation for the use of chemical weapons.

The US and its allies are looking beyond the divided UN Security Council to legitimise military action against Syria, trying to build a cohesive rationale for a strike and win broad international backing.

Prime Minister David Cameron recalled parliament to debate Britain’s response to a suspected chemical weapons attack in Syria, after his spokesman said plans were being drawn up for possible military action.

US allies were drafting plans for air strikes and other military action against Syria on Tuesday, as President Bashar al-Assad’s enemies vowed to punish a poison gas attack that Washington called a “moral obscenity”.

US Secretary of State John Kerry said on Tuesday that the evidence of a massive deadly chemical attack last week was “undeniable” and accused the Syrian government of trying to cover it up, signaling the United States was edging closer to a possible military response.

Barack Obama is unlikely to have much trouble forming a Nato coalition if Washington decides on military intervention in Syria after the alleged chemical weapons atrocities by the regime of Bashar al-Assad.

UN chemical weapons experts interviewed and took blood samples on Monday from victims of last week’s apparent poison gas attack in a rebel-held suburb of Syria’s capital, after the inspectors themselves survived sniper fire that hit their convoy.